Serge Giachetti takes photos of the raging Boulder Creek from the bridge near Arapahoe Avenue and Broadway on Sept. 13. A recent report indicates that some mountain creeks saw peak flows about five times the level of a 100-year flood during the September deluge. (Jeremy Papasso / Camera file photo)

The September rains and flood that battered much of Colorado's Front Range produced stream flows at some mountain locations up to approximately five times the rate of a 100-year flood, according to a recent report issued through the United States Department of Agriculture.

Hydrologist Steven Yochum, writing for the USDA's Natural Resources Conservation Service, said the report stemmed from work in which he was already involved performing flood predictions for some of the areas most severely impacted by the High Park Fire in Larimer County in June 2012.

Yochum and hydraulic engineer Daniel Moore focused on the September storm's effects on 15 mountain streams in northern Boulder County and Larimer County.

Their methodology limited them to water channels with slopes of greater than 1 percent, restricting their research area to foothills drainages, and excluding rivers and streambeds at lower elevations.

The highest peak flows recorded in the study were in the North Fork of the Big Thompson River upstream of Drake, at 18,400 cubic feet per second; the flow of Little Thompson River at Pinewood Springs was calculated at 14,600 cubic feet per second, and West Creek upstream of Glen Haven and Buckhorn Creek upstream of Masonville, which were both put at 11,000 cubic feet per second.

In Jamestown, where one resident was killed and the Boulder County Assessor's Office reported 26 properties were destroyed, the surge to Little James Creek upstream of the town, and to James Creek, were both calculated at about three times the rate of a 100-year flood.

The two locations where the researchers documented flow rates of five times the 100-year flood level were both in Larimer County — Fish Creek upstream of Lake Estes and Skin Gulch Creek upstream of Stove Prairie Road.

The extremely high flow at Skin Gulch Creek was attributed in the report to the 2012 High Park Fire, and the resulting severity of the charred landscape left behind.

High-altitude rainfall

From his work, Yochum said, a striking finding was how much rain clearly fell at a high elevation, which exacerbated the effects of the voluminous rain that also saturated much lower elevations.

"For some time (it) has been assumed that rainfall-dominated floods typically occur at elevations below 7,500 feet; above that elevation, it has generally been assumed that snowmelt rates are the primary driver of floods," Yochum wrote in an email.

"Documentation of this flood indicates that this assumption may need to be revisited, since large floods were documented from watersheds all or mostly above 7,500 feet during the Front Range Flood."

And in Yochum's and Moore's study, completed in December but which has not circulated outside of professional circles, it is stated that, "Within the limits of this data collection effort, these peak flow data indicate that the highest rainfall appears to have occurred in the vicinity of Jamestown, Twin Sisters and within the North Fork of the Big Thompson watershed."

Still calculating severity of flood

Calculations on the exact severity — and rarity — of the September flood are ongoing, but actually started when the rain was still falling.

Robert Kimbrough, associate director of the United States Geological Survey's Colorado Water Science Center, said on Sept. 12 that the flow rate on Boulder Creek at North 75th Street represented a 100-year flood — although his agency's preferred terminology is to call it a flood with a one-in-100 chance of occurring in any given year.

But Kimbrough and his colleagues have continued their work in the succeeding five months, calculating stream flows at 20 data collection points up and down the Front Range, in order to update and verify the peak flows and "recurrence intervals," or expected frequency of such a flood.

In numerous cases, the work of the USGS has been complicated by gages that were rendered inoperable or were simply washed away by the flood, requiring calculations to be drawn indirectly from data such as observation of high water marks left by the flood.

"We're making good progress. We should be able to complete most of the work by the end of February," Kimbrough said.

Kimbrough is one of several speakers scheduled to discuss the 2013 flood in detail on Feb. 27 during the 2013 Colorado Flood Forum to be sponsored by the Colorado Association of Stormwater and Floodplain Managers and Colorado State University. It is to be held at the Omni Interlocken Resort in Broomfield.

New coordinator pushes Buffs to work, play at level he expectsJim Leavitt has discovered this much about his new defense at Colorado: He has some talent with which to work, but his players need to put it in another gear. Full Story

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